Defence-in-depth is the term used by American political analyst Edward Luttwak (born 1942) to describe his theory of the defensive strategy employed by the Late Roman army in the third and fourth centuries AD.
Luttwak's Grand Strategy of the Roman Empire (1976) launched the thesis that in the third and early fourth centuries, the Imperial Roman army's defence strategy mutated from "forward defence" (or "preclusive defence") during the Principate era (30 BC-AD 284) to "defence-in-depth" in the fourth century. "Forward-" or "preclusive" defence aimed to neutralise external threats before they breached the Roman borders: the barbarian regions neighbouring the borders were envisaged as the theatres of operations. In contrast, "defence-in-depth" would not attempt to prevent incursions into Roman territory, but aimed to neutralise them on Roman soil - in effect turning border provinces into combat zones.
Scholarly opinion generally accepts "forward-defence" as a valid description of the Roman Empire's defensive posture during the Principate. But many specialists in Roman military history (which Luttwak is not) contest that this posture changed to Luttwak's "defence-in-depth" from 284 onwards. Described as "manifestly wrong" by the expert on Roman borders, C. R. Whittaker,Whittaker (1994) "defence-in-depth" has been criticised as incompatible with fourth-century Roman imperialist ideology (which remained expansionist), Roman strategic planning capabilities, with the evidence of fourth-century Roman historian Ammianus Marcellinus and with the vast corpus of excavation evidence from the Roman border regions.
Thesis According to this view, the Imperial Roman army had relied on neutralizing imminent barbarian incursions before they reached the imperial borders. This was achieved by stationing units (both legions and auxilia) right on the border and establishing and garrisoning strategic salients beyond the borders (such as the Agri Decumates in SW Germany). The response to any threat would thus be a pincer movement into barbarian territory: large infantry and cavalry forces from the border bases would immediately cross the border to intercept the coalescing enemy army; simultaneously the enemy would be attacked from behind by crack Roman cavalry (alae) advancing from the strategic salient(s).Luttwak (1976) Fig.3.3 This system obviously required first-rate intelligence of events in the barbarian borderlands, which was provided by a system of watch towers in the strategic salients and by continuous cross-border scouting operations (explorationes).
According to Luttwak, the forward defence system was always vulnerable to unusually large barbarian concentrations of forces, as the Roman army was too thinly spread along the enormous borders to deal with such threats. In addition, the lack of any reserves to the rear of the border meant that a barbarian force that successfully penetrated the perimeter defenses would have unchallenged ability to rampage deep into the empire before Roman reinforcements could arrive to intercept them.Luttwak (1976) 136 The first major challenge to forward defense was the great invasion of Germanic tribes (esp. Quadi and Marcomanni) across the Danube in 166–7, which began the Marcomannic Wars. The barbarians reached as far as Aquileia in northeastern Italy and were not finally expelled from the empire until 175. But the response of the imperial high command was not to change the forward defence strategy, but to reinforce it (by the founding of 2 new legions under Marcus Aurelius and 3 more under Septimius Severus and probably matching auxiliary forces).Luttwak (1976) 131-2 It was only after the catastrophic military crises of 251-71 that the Roman command under Diocletian turned to defence-in-depth: but only out of necessity, not conviction, as there were attempts to return to forward defence as late as Valentinian I (ruled 364–75)Luttwak (1976) 132 Forward defence had become simply too costly to maintain, especially with the emergence of a more powerful and expansionist Persian empire (the Sassanids) which required greater deployments in the East.
The essential feature of defence-in-depth, according to Luttwak, was the acceptance that the Roman frontier provinces themselves would become the main combat zone in operations against barbarian threats, rather than the barbarian lands across the border. Under this strategy, border forces would not attempt to repel a large incursion. Instead, they would retreat into fortified strongholds and wait for mobile forces (comitatenses) to arrive and intercept the invaders. Border forces would be substantially weaker than under forward defence, but their reduction in numbers (and quality) would be compensated by the establishment of much stronger fortifications to protect themselves: hence the abandonment of the old "playing-card" rectangular design of Roman fort. The new forts were so designed that they could only be taken with the use of siege engines (which barbarians generally lacked): square or even circular layout, much higher and thicker walls, wider perimeter berms and deeper ditches; projecting towers to allow enfilading fire; and location in more defensible points, such as hilltops. At the same time, many more small forts were established in the hinterland, especially along roads, to impose delays on the invaders. Also, fortified granaries were built to store food safely and deny supplies to the invaders. Finally, the civilian population of the province was protected by providing walls for all towns, many villages and even some villas (large country houses); some pre-Roman hillforts, long since abandoned, were re-occupied in the form of new Roman walled settlements.Luttwak (1976) Fig.3.2 The invading force would thus find itself in a region peppered with strongholds in enemy hands and where it could not easily get access to sufficient supplies. If the invaders ignored the strongholds and advanced, they risked sorties and attacks in the rear. If they attempted to besiege the strongholds, they would give the mobile troops valuable time to arrive. Overall, the aim of defence-in-depth was to provide an effective defence system at a sustainable cost, since defence-in-depth required much lower troop deployments than forward defence. To be more precise, the cost was transferred from general taxpayers to the people of the frontier provinces,Luttwak (1976) 137 especially the rural peasantry, who, for all the fortifications, would often see their family members killed or abducted, homes destroyed, livestock seized and crops burnt by marauding barbarians.
Critique Luttwak's work has been praised for its lucid analysis of, and insights into, issues regarding Roman military dispositions, and for stimulating much scholarly debate about these issues.Isaac (1992) 377 But the validity of his basic thesis has been strongly disputed by a number of scholars, especially in a powerful critique by B. Isaac, the author of the fundamental study of the Roman army in the East (1992).Mann (1979); F. Miller (1982); Isaac (1992) 372-418 The objections fall under two broad headings: (1) The Roman empire did not have the intelligence and planning capacity to sustain a "grand strategy" and in any case was not defensive in ideology or policy.Isaac (1992) 416(2) Defence-in-depth is not, in the main, consistent with the literary and archaeological evidence.Mann (1979) 180-1
Strategic planning Luttwak's thesis of an imperial grand strategy rests on a number of assumptions: (a) that the empire's strategic posture was basically defensive; (b) that Roman expansion and choice of borders were systematic and rational, with the main objective of securing defensible borders; (c) that the Roman government's primary concern was to ensure the security of its provincial subjects. But Isaac demonstrates that these assumptions are probably false and result from inappropriate application of modern concepts of international relations and military strategy to the ancient world.Isaac (1992) 373, 377 Isaac suggests that the empire was fundamentally aggressive both in ideology and military posture, up to and including the fourth century. This was demonstrated by the continued military operations and siting of fortifications well beyond the imperial borders.Isaac (1992) 387-93 The empire's expansion was determined mostly by the ambitions of emperors; and that the choice of borders, to the extent that they were planned at all, was more influenced by logistical considerations (e.g. rivers, that were critical conduits for supplies) rather than defensibility. Finally, the imperial government probably was far less concerned with the security of its subjects than would be a modern government.Isaac (1992) 393-4 Isaac shows that the empire did not develop the centralised military planning, or even accurate enough cartography, necessary to support grand strategy. Rome did not develop the equivalent of the centralised general staff of a modern army (and even less strategic studies institutes of the kind frequented by Luttwak). Emperors depended on the theatre military commanders (the provincial governors, later the magistri militum and duces) for all military intelligence.Isaac (1992) 378, 383, 401-6
Archaeological evidence There is also little unequivocal archaeological and literary evidence to support defence-in-depth. Luttwak's defence-in-depth hypothesis appears to rely on two basic features: (a) deepened fortified border zones: "It became necessary to build forts capable of sustained resistance, and these fortifications had to be built in depth, in order to protect internal lines of communication. Instead of a thin perimeter line on the edges of provincial territory, broad zones of military control had to be created..."Luttwak (1976) 159 "The thin line of auxiliary 'forts' and legionary 'fortresses' was gradually replaced by a much broader network of small fortified hard-points (in the hands of) scattered groups of static limitanei..."Luttwak (1976) 171 The hypothesis thus predicts the establishment of fortifications well into the interior of border provinces, rather than just a string of bases right on the border line; (b) the use of the comitatus praesentales (imperial escort armies) as interception forces to deal with incursions. Luttwak terminates his analysis in 350, before the establishment of the regional comitatus. The interception forces were thus the single large comitatus of Constantine, and, later, the 3 comitatus known from Ammianus to exist in 350 of Gaul, Illyricum and the East.Goldsworthy (2000) 172 But there are serious difficulties with both propositions.
(a) J.C. Mann points out that there is no evidence, either in the Notitia Dignitatum or in the archaeological record, that units along the Rhine or Danube were stationed in the border hinterlands.Mann (1979) 180 On the contrary, virtually all forts identified as built or occupied in the fourth century on the Danube lay on, very near or even beyond the river, strikingly similar to the second-century distribution.Scarre (1995) Map on p87Elton (1996) 157 and 159 (Fig 13)
Luttwak seizes on the situation in Palaestina Salutaris (mainly the former Arabia Petraea) province, which was dotted with forts all over, as an example of defence-in-depth.Luttwak (1976) 160 But here it cannot be proven that the defence system developed only in the fourth century. It may have dated from as early as the second century. In any case, Isaac shows that these "in-depth" forts were probably used for the purposes of internal security against rebels and brigands rather than defence against external threat.Isaac (1992) 198ff Indeed, such material as can be dated to Diocletian suggests that his reorganisation resulted in a massive reinforcement of linear defence along his newly built desert highway, the Strata Diocletiana.